For the purpose of developing an antibody drug, specifying a substance that causes an allergy, and the like, a protein is detected using fluorescent immunoassay, plasmon resonance, optical interference, and the like. In recent years, a method of detecting a protein using terahertz (hereinafter referred to as THz) waves has been proposed.
For example, when THz waves of different wavelengths are applied to a sample, THz waves of specified wavelengths are absorbed due to electromagnetic wave absorption by the sample, and the transmittance-frequency characteristics of the THz waves appear as a waveform specific to the physical properties of the sample. Therefore, by measuring the transmittance of the THz waves, a protein may be detected.
As a method of measuring transmittance of THz waves, THz time-domain spectroscopy (hereinafter referred to as THz-TDS) is known. In this THz-TDS, THz waves emitted from a THz wave generator are collected and applied onto a sample, and after that, THz waves which pass through the sample or THz waves which are reflected by the sample are collected onto a THz detector to be detected. FIG. 18 illustrates an imaging system formed of a THz-TDS system having a near-infrared femtosecond pulse laser 101 as a pump source.
A near-infrared femtosecond pulse laser emitted from the laser 101 for pumping is split by a beam splitter 102, and THz wave pulses are generated from a THz pulse generation side antenna 103 to which bias voltage 104 is applied. Generated THz waves 106 are collected by a paraboloidal mirror 105 to enter a sample unit 107. The incoming THz waves 106 enter a membrane sample 107b which is sandwiched between a high specific resistivity silicon 107a and a reflecting mirror 107c. The THz waves which pass through the membrane sample 107b are reflected by the reflecting mirror 107c. The sample unit 107 is fixed to an XY stage 108. Scanning in an X direction and in a Y direction enables acquirement of a two-dimensional image of the membrane sample 107b. 
In this case, as illustrated in FIG. 19, in the sample unit 107, reflection of the incoming THz wave pulses 106 is caused on a lower surface of the high specific resistivity silicon 107a (109a). Then, reflection is caused at an interface between the sample 107b and the high specific resistivity silicon 107a (109b). Further, a component which passes through the sample 107b is reflected by the reflecting minor 107c (109c). A part of the component which is reflected by the reflecting mirror 107c is reflected again by an upper surface of the high specific resistivity silicon 107a, and, after passing through the sample 107b again, is reflected by the reflecting mirror 107c (109d). These reflected waves 109 are collected by a THz wave receiving side antenna 110 to be detected.
In THz-TDS, for the purpose of acquiring a frequency spectrum by Fourier transform of a waveform of THz wave pulses, a current value at the instant at which THz waves that have passed through a time delay 111 reach the THz pulse receiving side antenna 110 is measured with an ammeter 112 and is recorded in a computer 113.
The above-mentioned membrane sample 107b realizes detection of a trace quantity of a biopolymer on a membrane filter which is a polymeric porous film. The membrane filter is permeable in the THz band, and is made of a material having a low refractive index because of the porosity. Therefore, when a trace quantity of a biopolymer attaches onto the membrane filter, the refractive index of that area is changed.
FIG. 20 shows frequency spectra acquired by Fourier transform of a temporal waveform acquired using the above-mentioned system. As shown in FIG. 20, even when only a trace quantity of streptavidin is coupled on the membrane filter, a frequency is observed at which a large change in transmittance is acquired by a shift of an interference waveform. By forming a two-dimensional image of data sliced at that frequency, the trace quantity of sample may be detected with high sensitivity. FIG. 21 illustrates an exemplary two-dimensional image generated in this way.